Alkalescent Chemical Silver Plating Solution

ABSTRACT

An alkalescent chemical silver electroless plating solution, which comprises: 0.01˜20 g/L silver ion or silver complex ion, 0.1˜150 g/L amine complexing agent, 0.1˜150 g/L amino acids complexing agent, and 0.1˜150 g/L polyhydroxy acids complexing agent. The alkalescent chemical silver plating solution provided by the present invention is able to overcome problems existing in acidic chemical silver plating processes commonly used at present. These problems include gnawing and corrosion of copper wires, lateral corrosion and difficulty of plating silver in blind holes, presence of solder ball voids and low strength of soldering. The silver layer plated by said silver plating solution possesses characteristics of high corrosion resistance, low contact resistance, no electromigration, high welding strength, and avoidance of bubbles produced in the solder when the plating pieces are being welded.

FIELD OF THE INVENTION

The present invention relates to a chemical silver plating solution, andmore specifically, to an alkalescent chemical silver plating solution.

BACKGROUND OF THE INVENTION

At present, acidic chemical silver plating solution is commonly used fora silver-plated process in the chemical plating industry; the mainproblems brought about by using the acidic formula and its correspondingprocess are:

1. Gnawing and Corrosion of Copper Wires

At present, a nitric acid system is the most popularly system in theworld used for the immersion silver. Nitric acid is a strong oxidizer,as well as a strong corrosive. It firstly corrodes parts with highstress in wires, thus causing stress corrosion, with the result that thewires become thin or are gnawed locally. Therefore, the immersionplating time must be strictly limited within 1 minute in a nitrate-basedchemical silver plating process. Otherwise, a phenomenon that the wiresbecome thin or become gnawed will appear. However, when the immersionplating time is only 1 minute, there is not enough time for reagents toget to the bottom of a blind hole which results in that the copper isexposed due to the bottom of the blind via not being plated by thesilver.

2. Very High Carbon Content in the Silver Plating Layer

For delaying the corrosion of the nitric acid on the copper andpromoting the reagents to get into the blind hole, a corrosion inhibitorthat easily forms a protective membrane on copper and a penetrant withgood permeability must be added into many nitrate-based chemical silverplating solutions. A corrosion inhibitor and penetrant can coprecipitatewith silver, thus the silver layer so-called ‘organic silver’ is formed,that is, the silver layer contains plenty of organics, or the silverlayer contains a high content of carbon. The content or purity of thesilver of the ‘organic silver’ layer of these companies is only 70%,that is, the content of carbon is up to about 30%. High carbon contentnot only influences the conductivity of the silver layer, but alsoinfluences the weldability, corrosion resistance of the silver layer andincreases high-frequency loss.

3. Void will be Formed in the Solder During Welding, thus Influences theWelding Strength

An acidic chemical silver layer contains a large amount of organics orcarbon, which can react with the oxygen in the air during hightemperature welding to produce carbon monoxide or carbon dioxide whichcan get into the melted solder. Because of the very high specificgravity of the melted solder, gas is hard to escape after getting intothe solder, and finally has been condensed in the solder and then voidsare formed. The thicker the silver plating layer is, the higher thetotal carbon content is, the more or larger the formed voids are, andmost of which are centralized at the upside close to the welding pad.

SUMMARY OF THE INVENTION

The primary objective of this invention is to overcome said problems ofthe prior art and provide a novel alkalescent chemical silver platingsolution that is capable of being widely used in the finishing processon the surface of a printed circuit board (PCB).

The alkalescent chemical silver plating solution provided in the presentinvention comprises components with the following amounts:

(1) Silver ion or Silver complex ion 0.01~20 g/L (2) Amine complexingagent 0.1~150 g/L (3) Amino acids complexing agent 0.1~150 g/L (4)Polyhydroxy acids complexing agent 0.1~150 g/L

Advantageously, said silver complex ion is at least one selected fromsilver ammonia complex ion, silver amine complex ion, silver-amino acidcomplex ion, silver-halide complex ion, silver-sulfite complex ion andsilver-thiosulfate complex ion.

Advantageously, said amine complexing agent is at least one selectedfrom ammonia, ammonium citrate tribasic, ammonium phosphate, ammoniumsulfate, ammonium nitrate, ammonium acetate, ammonium carbonate,methylamine, ethylamine, ethylenediamine, 1,2- propylenediamine, 1,3-propylenediamine, diethylene triamine, triethylenetetramine, triaminetriethylamine, imidazole, aminopyridine, aniline or phenylenediamine.

Advantageously, said amino acids complexing agent is at least oneselected from glycine, α-alanine, β-alanine, cystine, anthranilic acid,aspartic acid, glutamic acid, amidosulphonic acid, imido disulfonicacid, aminodiacetic Acid, nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA),hydroxyethylethylenediaminetriacetic acid (HEDTA) or aromatic amino acid(e.g. Pyridine-dicarboxylic acid).

Advantageously, said polyhydroxy acids complexing agent comprises atleast one selected from citric acid, tartaric acid, gluconic acid, malicacid, lactic acid, 1-hydroxy-ethylidene-1, 1-diphosphonic acid,sulfosalicylic acid, phthalic acid or their alkali metal salts orammonium salts.

Advantageously, the amount of said silver ion or silver complex ion is1˜80 g/L.

Advantageously, the amount of said amine complexing agent is 1˜80 g/L.

Advantageously, the amount of said amino acids complexing agent is 1˜80g/L.

Advantageously, the amount of said polyhydroxy acids complexing agent is1˜80 g/L.

Advantageously, the pH value of the chemical silver plating solution is8˜10, the plating temperature is 40˜70.

By employing the alkalescent chemical silver plating solution providedin the present invention, the following advantages are obtained:

a) The nitric acid free plating solution will not induce the problems ofgnawing and corroding the copper wires and lateral corrosion.

b) The plating solution is completely a complexing agent system thatdoes not contain corrosion inhibitor and penetrant. The silver layerobtained is a pure silver layer, which has excellent conductivity,anti-blushing ability and very low high frequency loss. Moreover, thelayer is easy to wash and has a low contact resistance and high wirebond strength.

c) There is no void in the solder ball during the welding for the puresilver layer, and the welding strength is high.

d) The plating solution is alkalescent with a pH value of 8˜10, thuswill not corrode the solder mask. The plating time can be set to 1˜5minutes to ensure all of the inside of a blind via is plated with silverwithout gnawing and corroding the copper wire and without lateralcorrosion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic diagram showing the copper wire being gnawed offafter being immersed in a nitrate-based silver plating solution for 3minutes;

FIG. 1 b is a schematic diagram showing the copper wire being laterallycorroded after being immersed in a nitrate-based silver plating solutionfor 3 minutes;

FIG. 2 is a pattern showing the XPS depth denudation of the acidicchemical silver plating layer;

FIG. 3 is a SEM micrograph of the void formed by the acidic chemicalsilver plating layer when welding;

FIG. 4 is a slice image of the chemical silver plating layer obtained byan alkalescent chemical silver plating solution provided by anembodiment of this invention after the welding.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS Embodiment 1

An alkalescent chemical silver plating solution according to a firstembodiment is described with the components and amounts as follows:

Silver nitrate 0.6 g/L  Triethylenetetramine 20 g/L Glycine 10 g/LCitric acid  5 g/L Deionized water remainder

In the chemical plating process, the pH value is adjusted to 7.8˜8.2 byammonia water. It merely needs about 5 minutes to plate the workpiece ata temperature of about 70° C. by using this silver plating solution.

Embodiment 2

An alkalescent chemical silver plating solution accordingly to a secondembodiment is described with the components and amounts as follows:

Ag⁺ ([Ag(NH₃)₂]⁺) 2 g/L EDTA 30 g/L  Ammonium nitrate 40 g/L  Lacticacid 2 g/L Deionized water remainder

In the chemical plating process, the pH value is adjusted to 8.2˜8.8 byammonia water. It merely needs about 1 minute to plate the workpiece ata temperature of about 50° C. by using this silver plating solution.

Embodiment 3

An alkalescent chemical silver plating solution according to a thirdembodiment is described with the components and amounts as follows:

Silver nitrate  6 g/L DTPA 40 g/L Ammonium citrate tribasic 30 g/LDeionized water remainder

In the chemical plating process, the pH value is adjusted to 8.8˜9.2 byammonia water. It merely needs about 0.5 minute to plate the workpieceat a temperature of about 45° C. by using this silver plating solution.

Embodiment 4

An alkalescent chemical silver plating solution according to a fourthembodiment is described with the components and amounts as follows:

Silver nitrate 10 g/L Ammonium carbonate 40 g/L Sulfosalicylic acid 40g/L Lactamine 40 g/L Deionized water remainder

In the chemical plating process, the pH value is adjusted to 7.8˜8.2 byammonia water. It merely needs about 3 minutes to plate the workpiece ata temperature of about 40° C. by using this silver plating solution.

Embodiment 5

An alkalescent chemical silver plating solution according to a fifthembodiment is described with the components and amounts as follows:

Silver sulfate  3 g/L Ammonium sulfate 20 g/L Imido disulfonic acid 30g/L Ammonium citrate  2 g/L Deionized water remainder

In the chemical plating process, the pH value is adjusted to 8.8˜9.2 byammonia water. It merely needs about 2 minutes to plate the workpiece ata temperature of about 55° C. by using this silver plating solution.

Embodiment 6

An alkalescent chemical silver plating solution according to a sixthembodiment is described with the components and amounts as follows:

Silver nitrate  1 g/L Ammonium phosphate 20 g/L Lactamine 60 g/LPhthalandione 10 g/L Deionized water remainder

In the chemical plating process, the pH value is adjusted to 9.8˜10.2 byammonia water. It merely needs about 3 minutes to plate the workpiece ata temperature of about 50° C. by using this silver plating solution.

Embodiment 7

An alkalescent chemical silver plating solution according to a seventhembodiment is described with the components and amounts as follows:

Silver amidosulphonic acid  8 g/L Amidosulphonic acid 30 g/L Ammoniumsulfate 50 g/L Tartaric acid 20 g/L Deionized water remainder

In the chemical plating process, the pH value is adjusted to 9.2˜9.8 byammonia water. It merely needs about 0.5 minute to plate the workpieceat a temperature of about 60° C. by using this silver plating solution.

Embodiment 8

An alkalescent chemical silver plating solution according to an eighthembodiment is described with the components and amounts as follows:

Silver nitrate 0.8 g/L  Picolinic acid 20 g/L Ammonium sulphonate 80 g/LGluconic acid  2 g/L Deionized water remainder

In the chemical plating process, the pH value is adjusted to 9.2˜9.8 byammonia water. It merely needs about 5 minutes to plate the workpiece ata temperature of about 60° C. by using this silver plating solution.

Embodiment 9

An alkalescent chemical silver plating solution according to a ninthembodiment is described with the components and amounts as follows:

Silver nitrate  3 g/L Ethylenediamine 10 g/L Sulfonicsalicyl acid 40 g/LAmidodiacetic acid  5 g/L Deionized water remainder

In the chemical plating process, the pH value is adjusted to 8.2˜8.8 byammonia water. It merely needs about 3 minutes to plate the workpiece ata temperature of about 50° C. by using this silver plating solution.

Embodiment 10

An alkalescent chemical silver plating solution according to a tenthembodiment is described with the components and amounts as follows:

Silver amidosulphonic acid  4 g/L Ammonium Nitrate 35 g/L Ethanolamine20 g/L Malic acid 10 g/L Deionized water remainder

In the chemical plating process, the pH value is adjusted to 9.8˜10.2 byammonia water. It merely needs about 1.5 minutes to plate the workpieceat a temperature of about 60° C. by using this silver plating solution.

The alkalescent chemical silver plating solution provided by thisinvention is capable of overcoming drawbacks existing in acidic chemicalsilver plating processes commonly used at present worldwide. Thesedrawbacks include gnawing and corrosion of copper wires, lateralcorrosion, difficulty of plating silver in a blind hole, solder ballvoids and low strength of the welding.

Comparative Cases:

1) Merchandise of MacDermid Inc., U.S. Pat. No. 6,200,451(2001)

Sterling ™ Silver A (75097)  5% Sterling ™ Silver B (75098) 10% Nitricacid  3% Pure water 83% Temperature 43-54° C.

Merchandise of MacDermid Inc., U.S. Pat. No. 6,200,451(2001)

Hydroxyethylenediamine tetracetic acid (HEDTA) 10 g/L Silver nitrate 2.4g/L  igepal Co730 (surfactant) 5.0 g/L  Imidazole 10 g/L Nitric acid 32ml/L

2) Enthone Inc. WO2006022835

Silver nitrate 1 g/l HEDTA 10 g/l  Benzimidazole 1 g/L EO/PO copolymer(surfactant) 1 g/l Nitric acid 0.98 g/L   pH 2The main problems which easily appeared in the above comparative casesare:

1. Gnawing and Corrosion of Copper Lines

Wires usually become thin or partly gnawed. Therefore, the time ofimmersion must be strictly limited within 1 minute in using aconcentrated nitrate-based immersion silver process. Otherwise, thecondition of wires becoming thin or being gnawed will appear. FIGS. 1 aand 1 b respectively show the conditions of the copper wire being gnawedand laterally corroded after being immersed in a nitrate-based silverplating solution for 3 minutes. However, when the time of immersion isonly 1 minute, there is not enough time for reagents to get to thebottom of a blind hole which results in copper exposure due to thebottom of the blind via not being plated by the silver.

2. Very High Carbon Content in the Silver Plating Layer

The silver layer obtained in the comparative cases are usually called“organic silver”, that is, the silver layer contains many organics, orthe silver layer contains a large amount (10-30%) of carbon. High carboncontent not only influences the conductivity of the silver layer, butalso influences the weldability, corrosion resistance of the silverlayer and increases high-frequency loss. FIG. 2 is a pattern showing theXPS depth denudation of the acidic chemical silver plating layer.

3. Void will be Formed in the Solder During Welding, thus Influences theWelding Strength

The large amount of organics or carbon in the silver layer can reactwith the oxygen in the air during high temperature welding to producecarbon monoxide or carbon dioxide which can get into the melted solder,become condensed in the solder and form voids. The thicker the silverplating layer is, the higher total carbon content is, the more or largerthe formed voids are, and most of which are centralized at the upsideclose to the welding pad. FIG. 3 is a SEM micrograph showing the voidformed by the acidic chemical silver plating layer when welding.

Compared to the above comparative cases, using an alkalescent chemicalsilver plating solution provided by an embodiment of the presentinvention results in overcoming problems existing in acidic chemicalsilver plating processes commonly used at present. These problemsinclude gnawing and corrosion of copper wires, lateral corrosion,difficulty of plating silver in a blind hole, solder ball voids and lowstrength of the welding. As shown in FIG. 4, there is a slice image ofthe chemical silver plating layer obtained by an alkalescent chemicalsilver plating solution provided by an embodiment of the presentinvention after the welding.

1. An alkalescent chemical silver plating solution, comprisingcomponents with following amounts: (1) Silver ion or Silver complex ion0.01~20 g/L  (2) Amine complexing agent 0.1~80 g/L (3) Amino acidscomplexing agent 0.1~80 g/L (4) Polyhydroxy acids complexing agent0.1~80 g/L


2. The alkalescent chemical silver plating solution according to claim1, wherein, said silver complex ion is at least one selected from silverammonia complex ion, silver amine complex ion, silver-amino acid complexion, silver-halide complex ion, silver-sulfite complex ion andsilver-thiosulfate complex ion.
 3. The alkalescent chemical silverplating solution according to claim 1, wherein, said amine complexingagent is at least one selected from ammonia, ammonium citrate tribasic,ammonium phosphate, ammonium sulfate, ammonium nitrate, ammoniumacetate, ammonium carbonate, methylamine, ethylamine, ethylenediamine,1,2- propylenediamine, 1,3- propylenediamine, diethylene triamine,triethylenetetramine, triamine triethylamine, imidazole, aminopyridine,aniline, or phenylenediamine.
 4. The alkalescent chemical silver platingsolution according to claim 1, wherein, said amino acids complexingagent is at least one selected from glycine, α-alanine, β-alanine,cystine, anthranilic acid, aspartic acid, glutamic acid, amidosulphonicacid, imido disulfonic acid, aminodiacetic Acid, nitrilotriacetic acid,ethylenediamine tetraacetic acid, diethylenetriaminepentaacetic acid,hydroxyethylethylenediaminetriacetic acid or aromatic amino acid.
 5. Thealkalescent chemical silver plating solution according to claim 1,wherein, said polyhydroxy acids complexing agent is at least oneselected from citric acid, tartaric acid, gluconic acid, malic acid,lactic acid, 1-hydroxy-ethylidene-1, 1-diphosphonic acid, sulfosalicylicacid, phthalic acid or their alkali metal salts or ammonium salts. 6.The alkalescent chemical silver plating solution according to claim 1 or2, wherein, the amount of said silver ion or silver complex ion is0.1˜50 g/L.
 7. The alkalescent chemical silver plating solutionaccording to claim 1, wherein, the pH value of the chemical silverplating solution is 8˜10, the plating temperature is 40˜70° C.